At present, a phase noise generated in a local oscillator is recognized as a problem that affects quality of high-frequency wireless digital communication.
In particular, since the phase noise greatly affects communication quality which is an essential part of a communication device, various methods for reducing the influence of the phase noise have been studied. For example, Patent Document 1 is known as the method for reducing the influence of the phase noise.
Further, the influence that the phase noise exerts on the communication quality becomes greater along with a higher frequency of recent communication devices.
Further, achievement of both an improvement in performance and reduction in price of a product for reducing the phase noise is required and, in view of this, a method for providing quality equivalent to or better than a currently-used product is required even in the case where a device that is more moderate in price but lower in performance than the currently-used device is used.
With reference to FIGS. 12 and 13, an outline of a transmitter/receiver used in high-frequency wireless communication will be described. FIG. 12 shows a transmitter part, and FIG. 13 shows a receiver part.
An orthogonal demodulator 9 (see FIG. 13) used for demodulating an RF signal into a baseband signal in this related art has a main function of performing carrier or clock reproduction in a transmission device performing wireless digital communication under the influence of phasing or the like which causes degradation of signals on a transmission path.
FIG. 12 and FIG. 13 show outlines of a transmitter and receiver used in high-frequency wireless digital communication, respectively. In the transmitter, a baseband signal (BB signal) is converted into an IF (Intermediate Frequency) signal by an orthogonal modulator 1 and a local oscillator 2. The baseband signal converted into the IF signal is further converted into RF (Radio Frequency) signal by a mixer 3 and a local oscillator 4. The baseband signal converted into the RF signal is then transmitted from an antenna 5.
The receiver receives the signal transmitted from the transmitter at an antenna 6. The received RF signal is converted into an IF signal by a mixer 7 and a local oscillator 8 and then converted into a baseband signal by an orthogonal demodulator 9 and a local oscillator 10.
In this configuration, a total of four oscillators are used. The phase noises generated from the four local oscillators are uncorrelated with one another.
Patent Document 1: International Publication Pamphlet No. WO 2004/001989